Atypical Aeromonas salmonicida Isolated from Diseased Eels

Abstract

Since 1979, occurrences of a new disease in cultured eels have been observed from late autumn to spring in Kagoshima and other prefectures. This disease has been tentatively called head ulcer disease because of its unique sign of ulcerative lesions formed on the head part of affected eels.
Bacterial isolations were made from the ulcerative lesions, liver and kidney of the diseased eels (Anguilla japonica) in Tokushima and Shizuoka Prefectures, and at Hiroshima University from 1981 to 1983. A bacterium, which formed small colonies on nutrient agar without pigment production after 48-72 h incubation at 20°C or 25°C, was isolated purely or dominantly from almost all the fish examined. From the morphological, biochemical, serological and genetical(GC value)properties, the organism was classified as atypical Aeromonas salmonicida.
The LD₅₀ of an isolate to Japanese eel was revealed less than 10² CUF/100 g fish by intramuscular injection, and subcutaneous injections into the head or nasal part of eels reproduced ulcerative lesions and exophthalmus. The mortality of artificially infected eels varied with ambient water temperature:10°C-30%, 15 and 20°C-100%, 25°C-70%, 30°C-0%.